1. Field of the Invention
The present invention relates to optical sensors and more particularly to an optical sensor for modulating light responsive to a parameter.
2. Summary of the Invention
A sensor which provides an output light signal representative of a parameter includes reflector means, waveguide means, lens means, and displacement means. The waveguide means transmits light toward the reflector means and receives light reflected from the reflector means to produce the light output signal. The lens means is positioned between the waveguide means and the reflector means. The lens means directs the light from the waveguide means to the reflector means, and focuses the light reflected by the reflector means to a light spot proximate the waveguide means. The light received by the waveguide means is a function of the relative positions of the waveguide means, the reflector means, and the lens means.
The displacement means causes relative movement of the reflector means or the lens means, or both, with respect to the waveguide means as a function of the parameter. This causes the amount of the light received by the waveguide means to vary as a function of the parameter.
In preferred embodiments, the sensor includes a sensor housing which supports at least one of the waveguide means, the reflector means, and the lens means in a fixed position to the housing. The displacement means is coupled to the housing and is responsive to the parameter to cause physical movement of at least one of the reflector means, the lens means, and the waveguide means with respect to the means which is supported in fixed relationship to the housing, so that relative movement of at least the reflector means or the lens means with respect to the waveguide means occurs as a function of the parameter.
In preferred embodiments, the waveguide means includes one or more optical fibers. The waveguide receives light from a light source and guides that light to a transmitting surface where the light is transmitted toward the reflector means. The waveguide has a receiving surface (which may be the same as the transmitting surface, depending upon whether one or more optical fibers are included in the waveguide) which receives the light from the reflector means. The received light is guided by the waveguide to a light detector, which produces an electrical output signal as a function of the intensity of the light output signal.
The lens means has an optical axis which is generally parallel to a path between the waveguide and the reflector means. The location of the light spot in a radial direction with respect to the optical axis is a function of the radial distance between the center of the transmitting surface and the optical axis. In a preferred embodiment, the displacement means causes relative radial movement of the lens means with respect to the waveguide, so that the location of a light spot formed by the focused light with respect to the receiver surface (and thus the intensity of the light received at the receiver surface) varies as a function of the parameter.
In a further preferred embodiment, the housing comprises a vortex shedding bar for inducing vortices in a flowing fluid. The displacement means comprises a motion transmitting assembly which is supported by the shedding bar within the vortices and which is physically repetitively displaced responsive to the vortices. The lens means comprises approximately a one-quarter pitch graded index lens having an index of refraction which varies as a function of radial distance from its optical axis. Light paths through a one-quarter pitch graded index lens have the shape of one-quarter wavelength of a sine wave signal. The lens is supported by and movable with the motion transmitting assembly and moves in the radial direction generally normal to the optical axis in response to movement caused by the vortices. The waveguide comprises a first and a second optical fiber disposed substantially equidistant and diametrically about the optical axis of the lens when the parameter has a predetermined value. The first fiber transmits light into a first end of the lens. The lens refracts the light and the reflector at a second end of the lens reflects the light back through the lens to focus the light to the light spot so that at least a portion of the reflected light spot is received by the second fiber. The position of the light spot and therefore the intensity of the light received by the second fiber is a function of the radial distance between the optical axis of the lens with respect to the first fiber. As the radial distance changes, the position of the light spot with respect to the second fiber varies (and thus the light intensity received by the second fiber varies), as a result of radial displacement of the lens. The amount of reflected light incident on the second fiber preferably varies as twice the radial displacement between the optical axis of the lens and the first fiber, thus providing a sensor which is very responsive to displacement.
In a further preferred sensor embodiment, the waveguide is a single multimode optical fiber which provides light to a first end of a one-half pitch graded index lens. The lens refracts the light and focuses the light to a light spot at the second end of the lens. A movable mirror is positioned adjacent the second end of the lens to reflect at least a portion of the focused light spot back into the lens and toward the optical fiber. The mirror is displaced with respect to the lens responsive to the parameter such that the amount of light reflected is a function of the parameter. The light is focused, and preferably has a small cross section at the mirror such that small displacements of the mirror result in substantially full range changes in the amount of light reflected. The reflected light is refracted through the lens and focused on the fiber. The received light is transmitted by the fiber to a beam splitter, which directs the received light to a detector. The detector provides an output signal as a function of the intensity of reflected light received by the fiber, and thus as a function of the parameter.
One benefit of the present invention is that use of a graded index lens provides efficient electromagnetic coupling between lens and waveguide such that coupling and transmission losses are minimized. A further benefit arises because the sensor is inherently single sided, which means that fibers transmit and receive light from one side of the lens. This simplifies manufacturing and, in the case of use as a vortex sensor, it permits both of the fibers to be disposed within a rigidly supported portion of the shedding bar such that fiber movement (which can affect light transmission characteristics and therefore intensity of the output signal) is minimized.